Residency · Residency · Cardiology

Transthoracic Echocardiography: Systematic Approach

Physics and Instrumentation

Ultrasound Fundamentals

Cardiac ultrasound imaging operates within a frequency range of 2 to 5 MHz for adult applications and 5 to 8 MHz for pediatric or subcostal imaging. The fundamental trade-off in ultrasound physics is that axial resolution improves proportionally with increasing frequency, but higher frequencies penetrate tissue less effectively, limiting depth visualization. Harmonic imaging has become standard practice in echocardiography, functioning by transmitting at the fundamental frequency and receiving at the second harmonic. This technique substantially reduces near-field artifact and improves endocardial border definition, which is critical for accurate ventricular function assessment. Frame rate is inversely related to sector width and depth, which means that narrowing the sector width is essential for optimizing M-mode and Doppler signal quality. The Nyquist limit, defined as one-half the pulse repetition frequency (PRF/2), represents the maximum Doppler shift that can be accurately detected. When this limit is exceeded, aliasing occurs in both pulsed-wave and color Doppler modalities, potentially leading to misinterpretation of flow velocities.

Doppler Modalities

Pulsed-wave (PW) Doppler provides range resolution, meaning it can interrogate flow velocities at a specific anatomic location. However, it is limited by a maximum measurable velocity of approximately 2 m/s at typical cardiac depths, beyond which aliasing occurs. Continuous-wave (CW) Doppler, by contrast, does not alias and can measure very high velocities, but it lacks range resolution and instead records the highest velocity encountered along the entire beam path. Color flow Doppler is essentially a two-dimensional representation of pulsed-wave Doppler, displayed using the BART convention where red indicates flow toward the transducer and blue indicates flow away. Variance or turbulence in the flow signal is typically displayed as a green or mosaic pattern overlaid on the color map. Tissue Doppler imaging (TDI) employs specialized filter settings that favor low-velocity, high-amplitude signals to record myocardial velocities rather than blood flow. The key measurements obtained from TDI at the mitral annulus include e-prime (early diastolic myocardial velocity), a-prime (late diastolic velocity), and s-prime (systolic velocity), each of which has important diagnostic implications for diastolic function and systolic performance.

Standard Imaging Windows and Views

Parasternal Long Axis (PLAX)

The parasternal long-axis view is obtained with the patient in the left lateral decubitus position, placing the transducer in the third or fourth left intercostal space with the orientation indicator directed toward the right shoulder. This window provides simultaneous visualization of the right ventricle anteriorly, the interventricular septum, the left ventricular cavity, the posterior wall, the mitral valve, the aortic valve, the left ventricular outflow tract, the aortic root, the left atrium, and the descending aorta running posterior to the left atrium. Key measurements obtained from this view include the left ventricular end-diastolic diameter (LVEDD, normal 3.5 to 5.7 cm), interventricular septal and posterior wall thickness (normal 0.6 to 1.1 cm), aortic root diameter (normal less than 3.7 cm), and the left atrial anteroposterior diameter (normal less than 4.0 cm). M-mode imaging through the left ventricle at the tips of the mitral leaflets is the standard approach for measuring wall thickness and chamber dimensions. The E-point septal separation (EPSS), which measures the distance from the anterior mitral leaflet to the septum during maximal early diastolic opening, is a rapid screening tool for left ventricular function: a value exceeding 7 mm suggests a reduced ejection fraction.

Parasternal Short Axis (PSAX)

The parasternal short-axis view is obtained by rotating the transducer 90 degrees clockwise from the PLAX position, with the indicator directed toward the left shoulder. A comprehensive assessment requires sweeping from the aortic valve level down to the apex. At the aortic valve level, the three cusps (right coronary, left coronary, and non-coronary) produce the characteristic "Mercedes-Benz" sign, and the interatrial septum, tricuspid valve, right ventricular outflow tract, pulmonic valve, and main pulmonary artery bifurcation are all visible. At the mitral valve level, the valve opening creates a "fish mouth" appearance that is useful for assessing commissural fusion in rheumatic mitral stenosis. The papillary muscle level displays both the posteromedial and anterolateral papillary muscles and is the optimal view for regional wall motion assessment, with six segments visible at this level corresponding to the left anterior descending, left circumflex, and right coronary artery territories. The apical level shows only the left ventricular cavity and is particularly useful for wall motion assessment and detection of apical thrombus.

Apical Four-Chamber (A4C)

The apical four-chamber view is obtained by placing the transducer at the cardiac apex with the indicator directed toward the patient's left side. This view simultaneously displays all four cardiac chambers. The normal right ventricle to left ventricle size ratio is less than 0.6:1 at end-diastole, and an RV that exceeds two-thirds the size of the LV suggests right ventricular dilation. Both the mitral and tricuspid valves are visible, and the tricuspid annulus is normally displaced apically by 5 to 10 mm relative to the mitral annulus. Absence of this normal offset should raise suspicion for an atrioventricular canal defect. Left ventricular volumes for ejection fraction calculation are measured using the Simpson biplane method by tracing endocardial borders in this view. The tricuspid annular plane systolic excursion (TAPSE) is measured using M-mode of the lateral tricuspid valve annulus, with a normal value of 17 mm or greater. Values below 17 mm indicate right ventricular systolic dysfunction. The apical five-chamber view is obtained by tilting the transducer anteriorly from the A4C position to bring the LVOT and aortic valve into the imaging plane.

Apical Two-Chamber (A2C)

The apical two-chamber view is obtained by rotating the transducer approximately 60 degrees counterclockwise from the four-chamber position. This view displays only the left atrium and left ventricle without right-sided structures, providing visualization of the anterior and inferior walls. It serves as the second orthogonal plane required for the Simpson biplane ejection fraction calculation and is essential for assessing inferior wall motion abnormalities in the right coronary artery territory and anterior wall abnormalities in the left anterior descending territory.

Apical Three-Chamber (A3C) / Apical Long Axis

Rotating further counterclockwise from the A2C position produces the apical three-chamber view, which is functionally equivalent to a "flipped" parasternal long-axis view. This projection visualizes the anteroseptal and inferolateral walls along with the aortic valve and LVOT, making it particularly useful for assessing the direction of aortic regurgitation jets and evaluating LVOT obstruction.

Subcostal Views

The subcostal approach is performed with the patient supine and knees bent, placing the transducer in the subxiphoid position. The subcostal four-chamber view provides the best visualization of the interatrial septum because the ultrasound beam is perpendicular to the septum in this orientation, making it the optimal view for evaluating atrial septal defects and patent foramen ovale with agitated saline contrast. The subcostal inferior vena cava view allows measurement of IVC diameter (normal less than 2.1 cm) and respiratory collapsibility with a sniff maneuver. Greater than 50% collapse estimates right atrial pressure at approximately 3 mmHg, less than 50% collapse suggests approximately 15 mmHg, and intermediate collapse corresponds to approximately 8 mmHg. The subcostal approach is often the only accessible window in mechanically ventilated patients, those with COPD, and post-surgical patients.

Suprasternal Notch

The suprasternal notch view is obtained by placing the transducer in the jugular notch to visualize the aortic arch, the brachiocephalic vessels, and the proximal descending aorta. This window is essential for assessing aortic coarctation (narrowing distal to the left subclavian artery), aortic dissection flaps, and patent ductus arteriosus.

<image> A comprehensive diagram showing the standard transthoracic echocardiography windows and transducer positions on a human torso. Illustrate a front view of the chest with ribs visible in light gray. Mark four main positions with colored dots: (1) Red dot at left 3rd-4th intercostal space parasternal for PLAX/PSAX with indicator direction arrows, (2) Blue dot at apex for A4C/A2C/A3C with rotation angles labeled, (3) Green dot at subxiphoid for subcostal views, (4) Orange dot at suprasternal notch. Adjacent to each dot, show a small representative echo image schematic with labeled chambers and structures. Include a legend showing transducer orientation indicator conventions. </image>

Left Ventricular Assessment

Systolic Function

Ejection fraction (EF) remains the cornerstone of left ventricular systolic function assessment. The Simpson biplane method, also known as the modified Simpson rule, calculates EF by tracing the endocardial border at end-diastole and end-systole in both the apical four-chamber and two-chamber views. Normal values are 52% or greater for males and 54% or greater for females. Visual estimation of EF by experienced readers is generally accurate within 5% of quantitative measurement and is typically reported in ranges: hyperdynamic (greater than 70%), normal (50 to 70%), mildly reduced (40 to 49%), moderately reduced (30 to 39%), and severely reduced (less than 30%). Fractional shortening, measured by M-mode, is calculated as (LVEDD minus LVESD) divided by LVEDD, multiplied by 100, with normal values between 25% and 45%. This measurement assumes symmetric contraction and is therefore unreliable in the presence of regional wall motion abnormalities.

Global longitudinal strain (GLS), derived from speckle-tracking echocardiography, has emerged as a more sensitive measure of systolic function than EF. Normal GLS ranges from -18% to -22%, with more negative values indicating better function. GLS has the ability to detect subclinical myocardial dysfunction before EF begins to decline, making it indispensable in cardio-oncology surveillance for early detection of chemotherapy-induced cardiotoxicity. Stroke volume is calculated by measuring the LVOT diameter in the parasternal long-axis view to determine the cross-sectional area (pi multiplied by the radius squared), then multiplying by the LVOT velocity-time integral obtained with pulsed-wave Doppler in the apical five-chamber view. Normal stroke volume ranges from 60 to 100 mL. Cardiac output is the product of stroke volume and heart rate, and cardiac index normalizes this to body surface area, with normal values between 2.5 and 4.0 L/min/m squared.

Diastolic Function Assessment (2016 ASE/EACVI Guidelines)

The assessment of diastolic function relies on four key variables: mitral annular e-prime velocity (both septal and lateral), the E/e-prime ratio, tricuspid regurgitation jet velocity, and left atrial volume index. Grade I diastolic dysfunction (impaired relaxation) is characterized by e-prime less than 7 cm/s at the septal annulus or less than 10 cm/s at the lateral annulus, an E/A ratio less than 0.8, E/e-prime less than 10, normal left atrial volume, and normal tricuspid regurgitation velocity. Grade II (pseudonormal pattern) features an E/A ratio between 0.8 and 2, E/e-prime in the indeterminate range of 10 to 14 or elevated above 14, left atrial volume index exceeding 34 mL/m squared, and tricuspid regurgitation velocity above 2.8 m/s. Grade III (restrictive pattern) is identified by an E/A ratio of 2 or greater, E/e-prime above 14, a short deceleration time of less than 160 ms, and elevated left atrial volume and tricuspid regurgitation velocity.

Regional Wall Motion Assessment

The American Society of Echocardiography 17-segment model divides the left ventricle into 6 basal segments, 6 mid-cavity segments, 4 apical segments, and an apical cap. Each segment is scored on a standardized scale:

<image> A detailed diagram of the ASE 17-segment left ventricular model displayed as a bullseye (polar map) plot. The center represents the apex (segment 17). The inner ring shows 4 apical segments (13-16), middle ring shows 6 mid-cavity segments (7-12), and outer ring shows 6 basal segments (1-6). Each segment should be labeled with its anatomical name (e.g., basal anterior, mid anteroseptal, apical lateral, etc.) and color-coded by coronary artery territory: LAD territory in red, RCA territory in blue, LCx territory in green, with overlap zones in striped patterns. Include a legend mapping colors to coronary arteries. Adjacent to the bullseye, show a small schematic of apical 4-chamber, 2-chamber, and 3-chamber views with corresponding segment numbers overlaid on the LV walls. </image>

Right Ventricular Assessment

Qualitative and Quantitative Parameters

Right ventricular assessment begins with obtaining an RV-focused apical four-chamber view by tilting the transducer medially to optimize visualization of the right ventricle. Quantitative measurements include the RV basal diameter (normal less than 4.1 cm), RV mid-cavity diameter (normal less than 3.5 cm), and RV longitudinal dimension (normal less than 8.3 cm). TAPSE, measured by M-mode of the lateral tricuspid valve annulus, should be 17 mm or greater and correlates well with overall RV ejection fraction. The RV s-prime velocity, measured by tissue Doppler at the lateral tricuspid valve annulus, should normally be 9.5 cm/s or greater. RV fractional area change (FAC) is calculated as (end-diastolic area minus end-systolic area) divided by end-diastolic area, multiplied by 100, with normal values of 35% or greater. RV free wall longitudinal strain is considered normal when more negative than -20%, and unlike TAPSE, it is relatively insensitive to angle and loading conditions. Three-dimensional RV ejection fraction is the most accurate noninvasive method for assessing RV function, with normal values of 45% or greater.

Pulmonary Artery Pressure Estimation

Right ventricular systolic pressure (RVSP) is estimated using the modified Bernoulli equation: RVSP equals 4 times the square of the peak tricuspid regurgitation velocity, plus the estimated right atrial pressure. This calculation assumes the absence of right ventricular outflow tract obstruction, and RVSP approximates pulmonary artery systolic pressure in the absence of pulmonic stenosis. Right atrial pressure is estimated from IVC assessment, with an IVC less than 2.1 cm that collapses more than 50% corresponding to 3 mmHg, an IVC greater than 2.1 cm with less than 50% collapse corresponding to 15 mmHg, and indeterminate values assigned 8 mmHg.

Valvular Assessment Essentials

Aortic Valve

Aortic stenosis severity assessment integrates peak velocity, mean gradient (derived from pulsed-wave Doppler through the LVOT and continuous-wave Doppler through the aortic valve), and aortic valve area calculated by the continuity equation. The continuity equation states that AVA equals the LVOT area multiplied by the LVOT VTI, divided by the AV VTI. Mild aortic stenosis is defined as AVA greater than 1.5 cm squared with a mean gradient less than 20 mmHg. Moderate stenosis is characterized by AVA between 1.0 and 1.5 cm squared with a mean gradient of 20 to 40 mmHg. Severe stenosis is diagnosed when AVA is less than 1.0 cm squared (or less than 0.6 cm squared/m squared when indexed), with a mean gradient of 40 mmHg or greater and a peak velocity of 4.0 m/s or greater.

Mitral Valve

Mitral regurgitation severity assessment requires integration of multiple parameters. Severe primary MR is characterized by a vena contracta of 7 mm or greater, an effective regurgitant orifice area (EROA) by PISA of 0.40 cm squared or greater, a regurgitant volume of 60 mL or greater, jet area to LA area ratio assessment, dense CW Doppler signal, and pulmonary vein flow reversal. For secondary MR, lower thresholds apply: severe EROA is 0.20 cm squared or greater and regurgitant volume is 30 mL or greater, reflecting the worse prognosis associated with even moderate regurgitant volumes in the setting of a diseased left ventricle.

Key Clinical Pearls

• Always measure LVOT diameter in PLAX at mid-systole, inner edge to inner edge, immediately proximal to the aortic valve -- a 1 mm error in LVOT diameter causes ~10% error in calculated AVA due to squaring
• Subcostal IVC assessment for RAP estimation should be performed during quiet respiration with brief inspiratory sniff; forced Valsalva or mechanical ventilation invalidate standard cutoffs
• GLS is the earliest marker of subclinical LV dysfunction in chemotherapy cardiotoxicity -- a > 15% relative decrease from baseline is clinically significant and should trigger cardio-oncology consultation
• In AF, average >= 5 cardiac cycles for all Doppler measurements; E/A ratio and deceleration time are unreliable; rely on E/e', TR velocity, and LA volume for diastolic assessment
• Apical foreshortening is the most common technical error leading to underestimation of LV volumes and overestimation of EF -- ensure the true apex is identified (no visible myocardium beyond the transducer)

References

• Lang RM, et al. Recommendations for Cardiac Chamber Quantification by Echocardiography in Adults: An Update from the ASE and EACVI. J Am Soc Echocardiogr. 2015;28:1-39.
• Nagueh SF, et al. Recommendations for the Evaluation of Left Ventricular Diastolic Function by Echocardiography: An Update from the ASE and EACVI. J Am Soc Echocardiogr. 2016;29:277-314.
• Zoghbi WA, et al. Recommendations for Noninvasive Evaluation of Native Valvular Regurgitation. J Am Soc Echocardiogr. 2017;30:303-371.
• Baumgartner H, et al. Echocardiographic Assessment of Valve Stenosis: EAE/ASE Recommendations. J Am Soc Echocardiogr. 2009;22:1-23.
• Rudski LG, et al. Guidelines for the Echocardiographic Assessment of the Right Heart in Adults. J Am Soc Echocardiogr. 2010;23:685-713.

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